Hello Alina,
Both are grammatically correct.
Either one can be used in the same situation.
Is it a mistake?
No, it's not a mistake.
No, it isn't a mistake.
No, it's not.
No, it isn't.
These are all good.
8 février 2016
1
3
Vous n'avez pas encore trouvé vos réponses ?
Écrivez vos questions et profitez de l'aide des locuteurs natifs !